1. Field of the Invention
The present invention generally relates to scanning probe microscopy (SPM) and more particularly to a scanning probe microscope tip with improved spatial resolution using chemically-synthesized nanoparticles.
2. Description of the Related Art
In scanning probe microscopy, specimens are imaged by scanning a sharp probe tip in close proximity to the specimen surface. Data acquired from the probe tip is plotted as a function of the location of the probe tip in the plane of the specimen surface.
Magnetic force microscopy is one of numerous scanning probe microscopy (SPM) techniques. Magnetic force microscopy is a probe to image magnetic fields in magnetic thin films. In the past, spatial resolution of magnetic details of approximately 10 nm has been achieved. Therefore, using a magnetic force microscope (MFM) is an effective tool to measure small magnetic fields arising from submicrometer scaled features.
The class of scanning probe microscopes further includes scanning tunneling microscopes (STM), near-field scanning optical microscopes (NSOM), scanning electrochemical microscopes (SECM) and atomic force microscopes (AFM). AFMs can observe the surface configuration of an insulating sample on an atomic scale.
The MFM consists of an AFM with a probe that contains a magnetic material. In a typical MFM system, a sharp magnetic tip is mounted on a cantilever force sensor. The tip is placed over a magnetic specimen at a separation of 10 nm to 500 nm from the surface of the specimen. Piezoelectric elements, capable of producing displacements as small as 0.01 nm, are used for positional control of the tip or specimen in any direction. The magnetic forces that act on the probe tip from the specimen cause a static deflection of the cantilever. This deflection is monitored by use of a laser detection system, for example, whereby the static deflection of the cantilever causes a corresponding displacement of a reflected laser light beam.
The scanning probe microscopy image is a composite of the effects of all the forces acting on the probe tip. In the absence of other field gradients, long-range Van der Waals forces attract the probe tip to the specimen surface and can be used to generate a topographic image of the surface of the specimen. Moreover, magnetic field gradients can be imaged if the probe tip has a sufficient magnetic dipole moment. The image may show only the magnetic field effects, a superposition of magnetic and topographic effects, or only topography, depending on the relative strength of the magnetic field and Van der Waals gradients as well as the material characteristics of the magnetic probe tip.
The material properties of the probe tip contribute to the increase or decrease in spatial resolution of an MFM. Various materials have been used for the magnetic probe tip in MFM. For example, use of a magnetized iron tip is described by Martin et al., “High-resolution Magnetic Imaging of Domains in TbFe by Force Microscopy”, Appl. Phys. Lett., Vol. 52, No. 3, Jan. 18, 1988, pp. 244-246. Also, the use of silicon tips coated with a film of magnetic material, such as NiFe or CoPtCr, in MFM is described by Grutter et al., “Magnetic Force Microscopy with Batch-fabricated Force Sensors”, J. Appl. Phys., Vol. 69, No. 8, Apr. 15, 1991, pp. 5883-5885. The standard method for forming an MFM sensor is to coat a standard AFM tip with magnetic material using standard thin-film deposition methods such as evaporation or sputtering. MFM sensors fabricated in this manner are limited in resolution by two main factors: 1) the film thickness of the magnetic coating layer increases the tip radius-of-curvature, thus decreasing resolution; and 2) the size of the magnetic domains in such a continuous thin-film also limits the resolution. Various methods have been attempted in an effort to overcome these problems. Patterning of the magnetic film deposited on the AFM tip can be done by ion-milling (S. H. Liou, IEEE Transactions on Magnetics, 35, 3989 (1999)). Alternatively, electron-beam lithography combined with shadow-evaporation of thin-films can produce regions of magnetic materials confined to the tip apex (G. D. Skidmore and E. D. Dahlberg, Applied Physics Letters 71, 3295 (1997), S. Y. Chou, S. Wei., P. Fischer, IEEE Transactions on Magnetics 30, 4485, (1994), M. Ruhrig et al., J. Appl. Phys. 79, 2913 (1996)). Both of these techniques are quite labor-intensive and are difficult to implement in a parallel manner.
Nanoparticles with diameters ranging from 2 nm to 20 nm can be made out of a wide variety of organic and inorganic materials (C. B. Murray, D. J. Norris, M. G. Bawendi, J. Am. Chem. Soc. 115, 8706 (1993), L. Brus in “Materials Chemistry: An Energy Discipline,” G. A. Ozin ed., 335 (ACS Sympos. Ser. No. 245, 1995)). Nanocrystals are a subclass of nanoparticles composed of well-characterized, crystalline cores and thin organic coats. Nanocrystals are monodisperse in terms of their size, internal structure (lattice), surface chemistry, and shape. Nanoparticles dispersed in liquids and nanoparticles deposited on solid substrates have provided much information on the submicroscopic properties of materials (A. P. Alivisatos, Science 271, 933 (1996)). Nanoparticles, and more specifically nanocrystals, attached to SPM tips could provide probes sensitive to a wide range of physical and chemical properties of a specimen, on a nanometer length-scale.
However, nanoparticles have not been used in SPM before because there did not exist a good method for attaching nanoparticles to SPM tips. Furthermore, magnetic nanoparticles have not been used in MFM before because, until recently, high-quality magnetic nanoparticles were unavailable (S. H. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser, Science 287, 1989 (2000)). Thus, there is a need for an improvement in the spatial resolution of a scanning probe microscope using a new material composition for constructing the probe tip.